Celebrity cook Nigella Lawson was "off her head" on drugs and let her assistants spend a fortune on a company credit card to keep quiet about it, a court has reportedly heard.

The court proceedings were reported by several British media, including Sky, The Daily Telegraph and The Independent.

However, the court also heard that the allegation - that Lawson used cocaine, cannabis and prescription pills daily for over a decade - was "totally scurrilous and untrue".

Two sisters who used to work as personal assistants to Lawson and her husband Charles Saatchi are on trial for fraud.

Italians Francesca Grillo, 41, and Elisabetto Grillo, 35, are accused of using the couple's credit card to spend £300,000 (NZ$593,000) on luxuries such as designer clothes and first-class air travel.

They deny the charges. Anthony Metzer QC, defending Elisabetto Grillo, said Saatchi was unaware Lawson took drugs during their marriage and only found out from defence lawyers in the fraud case.

"In a nutshell we submit that she had a guilty secret from her husband, her then-husband," Metzer told the court, according to the Daily Telegraph website.

"She did not want him to know of her use of cocaine and that is highly relevant to the defence case," Metzer said, according to the same article.

On Tuesday (local time), Isleworth Crown Court was read an email from Saatchi to Lawson in which he said the defendants "would get off" because Lawson was "so off your head on drugs".

"You allowed them to spend whatever they liked," the email - which was sent after the couple's separation - said.

It said that Mimi, Lawson's daughter from her first marriage, was also on drugs.

Judge Robin Johnson said Saatchi was astounded by the scale of drug use set out in the statements from the defendants.

He said he was allowing the email to be used in the court case because it addressed the question of whether the assistants had approval for their lavish spending.

Contacted by Reuters, Lawson's publicist Mark Hutchinson said: "As proceedings are live we can't comment at the moment." Under British law, it is legally risky to comment publicly about what is said during criminal court proceedings.

Lawson ended her 10-year marriage to Saatchi earlier this year after a newspaper published photos of the millionaire art dealer holding his wife by the throat during an argument at a Mayfair restaurant.

Saatchi called it a "playful tiff" but accepted a police caution for assault.

Metzer said the argument may have been in connection to the Grillo sisters.

"Our clients have been the innocent pawns in a rather unpleasant battle going on between Mr Saatchi, Miss Lawson and their lawyers," he said.

Prosecuting lawyer Jane Carpenter told the court that the drugs allegation was "a totally scurrilous account" that only emerged in the last few weeks, though the sisters were arrested more than a year ago and charged in March.